Radianthus malu ( Haddon & Shackleton, 1893 )

Titus, Benjamin M., Bennett-Smith, Morgan F., Chiodo, Tommaso & Rodríguez, Estefanía, 2024, The clownfish-hosting sea anemones (Anthozoa: Actiniaria): updated nomenclature, biogeography, and practical field guide., Zootaxa 5506 (1), pp. 1-34 : 22

Radianthus malu ( Haddon & Shackleton, 1893) ( Figure 12 ; Figure S6 )

The “malu ” or “delicate” sea anemone, Radianthus malu , is the least well-known clownfish-hosting sea anemone species. Individuals are generally small, but can reach 200mm in oral disc diameter, and are characterized by stubby irregularly shaped tentacles ( Figure 12 ; Figure S6 ). In general, tentacles are short (rarely exceeding 40mm in length) and bulbous, leading to a bluntly rounded tentacle tip. However, in some individuals, the bulbous swelling occurs lower on the tentacle leaving the tentacle tip to taper to a point ( Figure 12D ). As in R. magnifica , the tentacles often have a bright purple dot in the center of their bulbous tips ( Figure 12 A-C) but the tip can also be various shades of green ( Figure 12E ). Typical body and tentacle color for this species ranges from tan/cream to green and purple. Tentacles are typically sparse, leaving the oral disc visible in most individuals ( Figure 12B, E ). The oral disc is often radially striped ( Figure 12B, E ). Verrucae are present and form longitudinal rows on the upper portion of the column. Verrucae are strongly adhesive and often hold debris and sediment ( Figure 12C ). The lower portion of the column and pedal disc are buried in sediment and obscured from view.

This species occupies sand pockets in or adjacent to reefs as well as other calm sandy habitats. When disturbed, R. malu will retract completely into the sand. The reproductive mode for R. malu is thought to be sexual only. Individuals are typically found as solitary anemones but can be found adjacent to other anemones on occasion. This species is not always found hosting clownfishes, but when they do, they typically host juvenile fish. In life, this species can look like a cross between Entacmaea quadricolor and R. crispa and thus it is possible to confuse the three when making identifications in the field. Unlike E. quadricolor , R. malu has verrucae, and unlike R. crispa , the tentacles are stubby and sparse on the oral disc.

The geographic range of R. malu is centered in the Coral Triangle and extends South to Australia, North to Japan and West to the Southern Red Sea ( Figure 13 ). However, this species does occur east to the Marshall Islands and all the way to the Hawaiian Islands where it does not host clownfish. This is the only clownfish-hosting sea anemone species to make it as far east as Hawaii.

Figures

FIGURE 6. Representative images of the leathery sea anemone Radianthus crispa encompassing a broad range of geographic and phenotypic variation. A) Yellow individual hosting juvenile Amphiprion chrysopterus on a typical fore reef environment in the Pacific Ocean. Note the densely packed tentacles with tapered tips coming to a point (Moorea, French Polynesia). B) White individual with pink tentacle tips (Moorea, French Polynesia). C) Tan/Brown individual with less densely packed tentacles that curl slightly (Saudi Arabia, Red Sea). D) Macro photograph underneath oral disc. i = well-defined raised verrucae that are similarly colored to the surrounding column (Okinawa, Japan). E) Small individual in stereotypical sandy microhabitat with slightly curled tentacled tips and horizontally striated tentacle pattern (Gulf of Oman, United Arab Emirates). Photographs by Benjamin M. Titus and James Reimer.

FIGURE 12. Representative images of Radianthus malu encompassing a broad range of geographic and phenotypic variation. A) Two R. malu individuals exhibiting typical morphology and coloration for the species (Japanese Archipelago). B) Purple individual with sparse tentacles displaying visible oral disc (Kwajalein Atoll, Marshall Islands). C) Retracted individual exposing upper column with strongly adhesive verrucae. Note the debris (arrow) attached to verrucae (Japanese Archipelago). D) Individual with pointed tentacle tips hosting juvenile Amphiprion tricintus (Kwajalein Atoll, Marshall Islands). E) Brownish/ green individual with sparse tentacles and radially striped oral disc (Japanese Archipelago). Photographs by Scott Johnson and Takuma Fuji.

FIGURE 13. Confirmed geographic range of Radianthus malu in the Indo-West Pacific. Red dots represent species observations from the Global Biodiversity Information Facility (GBIF). Blue shaded area represents shallow water habitat (60 m bathymetry).